Ruby on Rails concern example: making an ActiveRecord item sortable. Adapted from: http://say26.com/show-us-your-production-code-1-concerns

item.rb

# app/models/item.rb
require 'concerns/sortable'

class Item < ActiveRecord::Base
  include Sortable

  attr_accessible :title
end

items_controller.rb

# app/controllers/items_controller.rb
class ItemsController < ApplicationController
  def index
    @items = Item.sorted(params[:sort], params[:direction])

    respond_to do |format|
      format.html
      format.json { render json: @items }
    end
  end
end

sortable.rb

# app/models/concerns/sortable.rb
require 'active_support/concern'

module Sortable
  extend ActiveSupport::Concern

  module ClassMethods
    def sorted(column, direction = nil)
      column = map_column(column)
      # Create a string to sort with (e.g. "title ASC")
      sort_string = [column, direction].reject(&:blank?).join(' ')
      order(sort_string)
    end

    private
      def map_column(column)
        mapping = {
          # param => table column
          'name' => 'title'
          # etc...
        }
        column = mapping[column]
      end
  end
end